The overall purpose of this project is to understand how viruses interact with their hosts in regulating viral development. The viruses used in these studies are the coli-bacteriophages lambda and Pl. The approach is to isolate a series of bacterial mutants which inhibit the action of specific phage functions. These bacterial mutants can then be analyzed to determine which host function(s) is necessary for a given phage function to be active. Using a number of selection procedures we have isolated a variety of bacterial mutants which inhibit phage lambda's growth. One such mutant isolated in this laboratory has been extensively studied. It inhibits lambda growth by inhibiting the action of the lambda N gene product, a function which permits extensive transcription of the lambda genome. These studies have led us to postulate a model of N action which states that the sites on the lambda genome for N recognition may differ from the sites of N action.